Menarche varies considerably from person to person, and thus no age cut off can be attributed. One of the primary tools used to follow a girl s progress through puberty is the Tanner staging system, which describes the pattern of development of the secondary sex characteristics. Tanner stage 2 corresponds to the beginning of breast development, which is the first visible sign of puberty in girls. Breast development is estrogen stimulated, and since ovulation cannot occur without estrogen, Tanner stage 2 will be a reliable marker for the definition of fertility.